With a magnetic disk apparatus, increasing track density for higher data storage density has meant that deformation of the disk due to temperature and humidity, or eccentricity of the disk center can no longer be neglected. Consequently, control for a magnetic head to cause it to follow the desired track closely must be implemented with the aid of tracking control by a servo method. In this form of control, a specific magnetic pattern (referred to as "servo information") is written on the magnetic disk in advance. Off-track error of the magnetic head on the track is detected by reading the servo information and position control of the magnetic head is performed for the head to follow the desired track on the basis of the off-track error.
In a small, fixed magnetic disk apparatus or a magnetic disk apparatus (for example, floppy disk apparatus) having an interchangeable storage medium containing a small number of magnetic disks, it is difficult to provide an exclusive servo disk and therefore a so-called data-surface servo method has been employed, in which the servo information is written on the same magnetic disk upon which the data is stored. Also, the following sector servo method has been employed with interchangeable storage media. A data track is divided into sectors S(1) to S(n) as shown in FIG. 1. Storing and reading the information is effected on a sector-by-sector basis with each sector being arranged on the storage track to have an ID field for specifying the position of that sector and a DATA field for storing data. Adjacent fields are separated by a gap G. The servo information is written in the respective sectors.
The sector servo method involves several problems with the magnetic disk. For example, the sector servo method requires provision of servo information at predetermined sectors and servo information fields on the respective tracks will have to be increased because the servo information should be read very frequently to enable the magnetic head to follow the desired track at high speed and with high accuracy. However, increasing the number of servo information fields decreases the area of the DATA field for data storage.
One way of assuring the required amount of servo information without decreasing the size of the DATA field would be to store the servo information in a deep layer of the magnetic disk. Generally, storing different magnetic patterns on the surface portion and a deep portion of the magnetic disk can be accomplished by first writing into a deep portion with a wide-gap magnetic head and then overwriting the same track field with a narrow-gap magnetic head. In addition, when reading the information by means of the magnetic head, the servo information and the data can be separated with filters from the information read by the magnetic head. The position at which the servo information is stored is not described as being important in prior art disk type magnetic storage media in which the servo information is stored in deep portions of the respective tracks as mentioned above. Nonetheless, writing servo information into the deep portion of the data field having a large area in a sector may disturb readout of the servo information through the magnetic head.
When writing new data on the data field of the respective sectors, the data is usually overwritten on the same field. Variations in the gap of the magnetic head cause different distribution functions of the magnetic field and result in variations in the thickness of magnetized portions of the servo information that is left out of the deep portion. The reproduced output level of the servo information for tracking the desired track will vary and cause unreliable readout of the servo information. Consequently, precise detection of the off-track error will not be obtained. This is particularly true with a medium-interchangeable type of apparatus (for example, floppy disk apparatus) under the situation where the data is first stored into a disk using one disk drive and later reproduced from the same disk using another disk drive. Variations of the gaps of the magnetic heads in the respective disk drives will cause variations in the precision of detecting off-track error in the respective disk drives.
Head tracking control of prior art disk storage and reproduction drives described previously has the following shortcomings. Conventionally, when performing tracking control of the head described above, the servo information is read by means of the read/write head relative to the desired track. The off-track error signal is supplied to a microprocessor after A/D conversion. The microprocessor calculates and outputs position correction data for the read/write head on the basis of the off-track error signal. The position correction signal is supplied to a head drive circuit as a control signal through a D/A conversion circuit. The head drive circuit controls the position of the read/write head on the basis of the control signal so that the read/write head follows the desired track.
The above-mentioned conventional tracking control apparatus, cannot ensure track-following with high precision for several reasons. For example, as shown in FIG. 2, if eccentricity of .DELTA.d is caused by disk chucking error, the position of the desired track varies as depicted by the characteristic Q in FIG. 3 relative to the position if there is no eccentricity.
The phase-lag in the motion of the read/write head with respect to the change in position of the track is caused by the required process time from the reading of the servo information by the read/write head to the completion of calculating by the microprocessor and subsequent operation of an actuator. Specifically, when off-track error .DELTA.X relative to the desired track is caused at time t.sub.0 in FIG. 3, signal processing is performed to correct the off-position error. When the off-track error .DELTA.X is corrected after a certain process time, the variation from the desired track will have changed. Thus, if the desired track varies with characteristic Q in FIG. 3, then the read/write head will follow with characteristic Q' in FIG. 3, causing the track-following capability to be degraded by the phase lag.
In the case where the necessary information is written at the same time as the aforementioned tracking control is performed, identification information written in an ID field of the respective sector is read by means of the read/write head. When the sector is determined on the basis of the ID information to be the sector into which data is to be written, the necessary information is written into the DATA field of that sector. The data is written through an overwrite operation in which even though the DATA field has already been written, additional data is written over the previous data. This is a further shortcoming of conventional disk devices.
A conventional magnetic disk apparatus as thus far described not only has a limitation in implementing high storage density on a magnetic disk, but cannot ensure a stable condition for writing information. This problem is compounded because in data writing operations a magnetic head with a narrow gap is used to improve reproduction resolution in high density storage and the data that has been written previously may not be erased completely because the storage magnetic field does not reach deeply into the disk when writing data with the narrow gap magnetic head. Thus, a stable data writing operation cannot be assured.